The endocrine mechanisms involved in term and preterm delivery in primates, including pregnant women, are poorly understood. In the term monkey, fetal plasma androgen concentration rises to two hundred times the maternal concentration which remains unchanged. Placental conversion of androgen to estrogen results in increased maternal plasma estrogen concentration at term in both pregnant nonhuman primates and women. In the present study, continuous infusion of androstenedione to 0.8 gestation monkeys resulted in the premature occurrence of labor–type myometrial activity and increases in maternal plasma estrogen, oxytocin and amnion fibronectin concentrations similar to those measured at normal–term labor. Androstenedione induction of these normal–term biochemical and endocrine changes accompanied by fetal membrane rupture, cervical dilatation and live delivery provides a rich opportunity to study the molecular and physiological mechanisms of both term and preterm labor in primates.